The present invention relates to adjusting a transmit power spectrum of a transmit/receive device of a communication network, as it is, for example, required in DSL technology (DSL=digital subscriber line).
Due to the plurality of pairs of wires in a telephone cable from a central office (CO) to connected subscribers or subscriber devices, respectively, crosstalk occurs despite a direct current insulation of the pairs of wires, both near-end crosstalk (NEXT) and also far-end crosstalk (FEXT). The same result from capacitive or inductive couplings. By twisting the individual pairs of wires of a cable, this crosstalk is minimized so far that it may be neglected in the voice band. In data transmission technologies, like e.g. DSL technology, however, apart from the voice band further frequency ranges are used, in which this crosstalk has a very strong effect on the possible range and transmission speed.
Near-end crosstalk means, that a transmitter on the “near side” of a receive device, i.e. both receive device and also transmitter are part of different subscriber terminal devices or both receive device and also transmitter are part of the central office, overcouples transmit signal portions into the receive branch of the receive device.
In contrast to that, far-end crosstalk means, that, in addition to the receive signal determined for the same, sent from a far-end first transmitter, i.e. first transmitter and receiver are on different sides of subscriber and central office side, a receiver receives portions of a second transmit signal sent from a far-end second transmitter, i.e. also second transmitter and receiver are on different sides.
In an xDSL network, wherein “x” stands for different implementations of DSL technology, the subscriber devices (CPE=customer premises equipment) are typically in a different distance from the central office (CO). A so called near-far problem results from this. If all subscriber devices of the network transmitted with the same transmit power spectral density (TX-PSD, transmit power spectral density), the CPEs closer to the CO would respectively couple a high amount of crosstalk into the lines of the CPE further away from the CO and thus cause differently strong interference power spectral densities of the individual subscribers.
It is sometimes the case, that in an xDSL network ADSL lines (ADSL=asymmetric digital subscriber line) are laid together with VDSL lines (VDSL=very high speed digital subscriber line) in cable bundles. Due to higher data rates of VDSL systems, VDSL central offices are generally installed closer to the corresponding connected VDSL subscribers than comparable ADSL central offices with regard to their connected ADSL subscribers. The lines respectively starting from the central offices are combined in cable bundles and led in parallel at the end of the subscriber side for a few hundred meters, so that, for example, newly installed VDSL systems generate interferences by cross-talk on existing ADSL lines.
For this reason, within the scope of VDSL standardization (VDSL=very high speed digital subscriber line)(see ITU-Standards G.993.1, G.993.2, G.997, ANSI-Standard T1.424-2004, ETSI-Standard TS101-270), a so-called Downstream Power Back-Off (DPBO) is defined. This is a method for protecting existing ADSL lines. It is the aim of the DPBO to form a transmit power spectral density (TX-PSD) at the VDSL central office side, such that the interference power spectral density caused by a VDSL wire pair on an ADSL subscriber side by far-end cross-talk comprises the same value as the FEXT-PSD generated at the same ADSL subscriber by a neighboring ADSL wire pair.
The indicated standards define a method which approximates the request for a respectively equal interference power density of the wire pairs at the CO by realizing the respectively equal receive power spectral density (RX-PSD). By this approximation, subscribers on shorter lines cause a lower FEXT-PSD than subscribers on longer lines. This disadvantage may be avoided by a correction value. The correction value depends on an electrical length or attenuation (EL, in dB), respectively, of the ADSL lines on a line length according to the distance between the ADSL central office and the VDSL central office, and on the electrical length or attenuation (CL, in dB), respectively, of the VDSL lines between the VDSL central office and the VDSL subscribers. To the TX-PSD in dBm/Hz determined according to the standardized method, a correction value
                              a          corr                =                              10            ·                                          log                10                            ⁡                              (                                  1                  +                                      EL                    CL                                                  )                                              ⁢          dB                                    (        1        )            is added. The line attenuations or electrical lengths EL and CL, respectively, may alternatively also be indicated in meters, as the ratio EL/CL does not depend on that.
The value EL is generally known to a VDSL network operator and may be taken, for example, from a database for the configuration of the VDSL system. The value CL, however, is different for each VDSL terminal line and thus unknown in the configuration of the VDSL system. Usually, here a suitable mean value is selected. If the selected mean value of the electrical length CL is greater than an actual electrical length or line attenuation, respectively, for a VDSL subscriber, then the correction value acorr for the VDSL subscriber is accordingly too low. If, on the other hand, the mean value of CL is smaller than the actual electrical length or line attenuation, respectively, of a subscriber line, then the associated correction value acorr is too high, i.e. independent of EL. That means, that in line bundles mainly including short VDSL lines, via the VDSL lines a clearly lower data rate is transmitted than would be possible with an exact knowledge of the subscriber-specific (VDSL subscribers) values for CL. Further, this means, on the other hand, that in line bundles mainly including long VDSL lines, the VDSL lines cause clearly more cross-talk than ADSL lines running in parallel in the line bundles and thus an ADSL system is only insufficiently protected.
This means that a request for equal downstream conditions with the use of a mean value for CL for calculating the correction value acorr may only be achieved insufficiently.